Usually, molded materials made of polyolefins like polyethylene, polypropylene, polymers like polymethylmethacrylate, polycarbonate, etc. are not only lighter in weight and superior in resistance to impact in comparison with materials made of metal, glass, etc. but also have various advantages as being inexpensive and easily fabricated. Accordingly, they are used widely in many fields of motorcars, motorcycles, domestic electrical goods, general merchandise, etc. in place of those materials. However, these molded polymers are, when compared with metals or glass, not as high in surface hardness and not so resistant to scratch and abrasion such that they are defective in being susceptible to scratches on the surface. For example, since molded polymers are apt to be damaged on the surface by contacting, colliding, scratching, etc. during their transportation, usages, fixation of parts thereon, etc., molded polymers are limited in their application due to the defect in surface characteristics.
For the purpose of obviating the above mentioned defects of molded polymers, various methods have been proposed, and most of the methods are coating surfaces of molded polymers with layers of film composed of crosslinked curable resins. Among the film forming elements, proposed resins or resin forming components include silicone monomers or their compositions with various polymers, resin compositions of methylolmelamine with hardening components, and compositions of polyfunctional acrylic carbonic acid ester derivatives or their compositions with other polymer components.
Among the various polymer materials indicated above, with respect to the polyolefins, when a coated layer composed of a film forming element is formed on the surface of a molded polyolefin like polyethylene, polypropylene, etc., the coated molded material has a defect of having a coated layer readily peelable due to the poor adherence between the coated layer and polyolefin substrate. Methods for obviating the defects on the surface of molded polyolefin include surface treatments with corona discharging, coating with primers, etc. However, the surface treatment hardly improves the adhesion between the polyolefin substrate and the coated film composed of curable resins to the extent of being responsive to practical use. Among the above mentioned film forming elements, silicone film forming elements are expensive and are not preferred economically.
Among the aforesaid film forming elements, various types of compounds belonging to polyfunctional acrylic carbonic acid ester derivatives are proposed. For example, such types of compounds as poly(meth)acrylates of alkanepolyols, poly (meth)acrylates of polyoxyalkyleneglycols, poly (meth)acrylates of aromatic (phenolic) polyhydroxyl compounds, etc. are proposed for usage in film forming elements. In this specification, acrylates and methacrylates represent (meth)acrylates in the abbreviation form and hereinafter the same abbreviations are used. When coated films are formed on molded polyolefin substrates by the sole use of these polyfunctional acrylic carbonic acid ester derivatives as film forming elements, the coated resin shows shortcomings in the curing characteristics including the curing speed of the coated resin under atmosphere, and in the cured film properties including surface hardness, resistance to scratching, resistance to abrasion, flexibility, surface luster, resistance to heat, resistance to water, resistance to solvent, resistance to weathering, adhesion to substrate, etc., which made the coated film unsatisfactory for practical industrial usage. Combinations of more than two kinds of compounds belonging to the film forming element were put to the test to obviate these defects, however, the trial created new difficulties in coating surfaces of polyolefin substrates, though the drawbacks of sole use were remedied to some extent.
In these years, in accordance with the progress of informative society, need for large volume information recording media has increased. Optical recording media (opto-magnetic recording media) possess the advantages of being able to rewrite data, a large capacity, portable, capable of non-contact reading out and resistant to scratch or dust. Accordingly, applications of optical recording media have being expanded not only in auxiliary memories of computers but also in memories for various communication equipment like telephones, facsimiles, etc. as well as media for audio equipment. Previously, the opto-magnetic recording had a disadvantage of being unable to overwrite, and for this purpose of overcoming the defect, so called magnetic modulation recording has become widely employed recently in which the magnetic field for recording is reversed in conformity with recording signals during the recording. In the case of conducting a high frequency recording with the recording method, the floating head system is employed so as a magnet head will be able to follow the vibrating surface of the medium rotating in high speed.
As for the motion of a floating type head, the head is in a floating state while the medium keeps rotating, but is in a state of contact with the medium while the medium is stopped or at the start of rotation, and the phenomenon is referred to as CSS (Contact Start and Stop). Under the CSS motion, friction between the medium and head is produced, and repeated friction has a possibility of causing breakdown of medium or breaking of head (head-crush), which deteriorates the credibility of equipment so remarkably. Accordingly, magnetic heads and recording media being resistant to friction at Contact Start and Stop (CSS resistant) are required, and surface coatings for recording media are especially desired to be superior in resistant to friction.
For satisfying the requests, various methods of coating with crosslinking curable type resin information recording media like opto-magnetic media, on the surface of recording layer (over-coat) or the surface opposite to recording layer (top-coat), have been proposed.
For top-coats on the surface opposite to recording layer of information recording media, superior antistatic characteristics are requested in addition to the above-mentioned properties for coated film. It has been known that an addition of surface active agents to a resin composition mainly composed of polyfunctional acrylic monomers increases the antistatic effects. However, since those surfactants added heretofore show tendency of coming out as a resinous liquid on the surface of cured resin as known as bleed out, they are not suitable for usage in top-coats of optical disks, etc. Further, satisfactory antistatic property causes such problems as opaqueness of coated films and decrease in adhesion to substrates.
Surface coatings highly resistant to abrasion used for over-coats or top-coats of information recording media are generally hard and inferior in adhesion to other layers. Especially, since opto-magnetic disks contain reflecting layers composed of metal like aluminum, nickel alloys, etc., or thermoconductive layers composed of aluminum alloys, etc., adhesion between these metallic layers and the abrasion resistant surface coating is an important problem.